Vegetable oils are biodegradable and unlike petroleum based lubricants, vegetable oils are derived from renewable resources. These characteristics make them excellent base stocks for the formulation of environmentally friendly lubricants. However, one major limitation of vegetable oils is their poor resistance to oxidative and thermal breakdown even in the presence of oxidation and corrosion inhibitors.
In U.S. Pat. No. 4,880,551, there are provided synergistic antioxidant compositions containing (a) 1-[di(4-octylphenyl)aminomethyl]tolutriazole and (b) 2,6-di-t-butyl-4-secbutylphenol, 2,6-di-t-butyl-methylphenol, and butylated phenol mixture. Another aspect of that disclosure concerns a lubricating composition comprising a major portion of mineral oil or synthetic lubricating oil, fluid or grease and 0.1 to 5.0 percent of aforementioned antioxidant composition. However, U.S. Pat. No. 4,880,551 does not consider lubricating compositions based on vegetable oils which are neither mineral nor synthetic in nature.
U.S. Pat. No. 4,880,551 also states that lubricating compositions may further contain extreme pressure agents and antiwear additives among other additives types. Work presented herein confirms that the antioxidant combination in U.S. Pat. No. 4,880,551 is very effective in providing thermal and oxidative stability and corrosion resistance to vegetable oil. However, the addition of phosphorus based or phosphorus/sulfur based ashless antiwear additives were antagonistic on these properties with the surprising exception of triphenylphosphorothionate (TPPT). In addition, antiwear protection provided by TPPT used at the inventive concentration exceeded that of other antiwear additives.
U.S. Pat. No. 5,538,654 discloses lubricating compositions comprised of (A) major amount of a genetically modified vegetable oil and minor amounts of (B) phenolic antioxidant and (C) TPPT in which (A):(B):(C) weight ratio are (94-99.9):(0.05-5):(0.05-1). However, the reference teaches that the upper limit for TPPT is 1%; and therefore does not foresee that the use of TPPT at 1.5 or higher weight percent would improve antiwear protection, or that 1-[di(phenyl)aminomethyl]tolutriazole acts synergistically with TPPT to achieve the desired antiwear protection, as well as acting to prevent detrimental effects on thermal stability and corrosion properties.
Thus, the present invention relates to lubricant compositions comprising a major amount of vegetable oil, and minor amounts of TPPT, phenolic antioxidant, 1-[di(phenyl)aminomethyl]tolutriazole, and ashless rust inhibitor. The invention also relates to an additive composition comprising TPPT, phenolic antioxidants, phenyl amino derivatives of benzo- or tolutriazole, and ashless rust inhibitor, which affords excellent thermal and oxidative stability, corrosion resistance, and antiwear properties when used in combination with vegetable oil based lubricant compositions. In one embodiment of the invention, the additive composition and the lubricating composition containing same are free or substantially free of phosphorus- or sulfur-based ashless antiwear additives, such as ashless dialkyldithiophosphate and amine phosphate antiwear additives, with the exception of TPPT.